Recently, the appearance of powerful rare-earth magnets on the market has enabled the development of small, high efficiency, high power permanent magnet-type synchronous motors that use rare-earth magnets in the rotors. These motors are used as motive sources and control driver sources in a variety of industries. In order to use such motors at high power, there have been increases in the electric current capacities of inverters for motors.
One example of a conventional electric motor system is illustrated in FIG. 7. This electric motor system has two three-phase coil sets with distributed windings with branched connections, and two inverters, connected in series with the individual coils. Each of the inverters is provided with six insulated gate bipolar transistors (IGBTs) that are in complementary connection. The respective IGBTs are driven by gate drivers controlled using a PWM method, to provide three-phase AC power from the IGBTs through electric current parallel reactors L to the two coils of the electric motor. Another inverter is structured in the same manner, to drive in parallel, through two inverters, the respective three-phase coil sets that are connected in parallel. That is, in an electric motor with this type of structure, two inverters are connected in parallel to two three-phase coil sets. In this type of structure, an electric current parallel reactor L is indispensable for mitigating imbalance between the respective electric currents from the two inverters.
FIG. 8 illustrates another conventional electric motor system. This electric motor system has an electric motor having a single three-phase coil set that has a branched connection, and inverters 71, 72, and 73, that have IGBTs that are connected in parallel. This electric motor system enables the supply of a large electric current through connecting in parallel relatively inexpensive below-current IGBTs. However, because the IGBTs are connected in parallel, balancing the electric current between the IGBTs is difficult, and typically it is necessary to have circuit designs that take into account a derating of between 10 and 30% (as it is necessary to use a device that has a rating that has a margin relative to the power use). Because of this, it is necessary to use IGBTs that have large rated powers relative to the maximum electric current values required by the electric motor, increasing the cost of manufacturing the inverters.
Japanese Patent Application Publication JP-A-9-331694 discloses an induction motor wherein a high number of multiply split coils are formed by splitting coils for each phase, and multiple inverter primary circuits that are capable of applying multi-phase alternating current power individually to the split multi-phase coils are provided. This motor provides a high power inverter motor without requiring high power switching elements, which are relatively expensive when compared to low power ones.
Japanese Patent Application Publication JP-A-7-298685 discloses a system for driving a 6-phase induction motor using two three-phase PWM inverters. The 6-phase induction motor has six phase coils u1, y1, w1, x1, v1, and z1, where the coils u1, w1, and v1 form a three-phase winding W1, and the coils y1, x1, and z1 form a three-phase winding W2. The two three-phase PWM inverters produce voltages with waveforms with a 180° phase difference, which are connected to the respective three-phase windings W1 and W2. Thus a high power driving system is disclosed that provides either an in-phase or anti-phase symmetrical voltage waveform to each winding, where two different windings are connected to multiple inverters by forming a six-phase induction motor wherein the windings that are formed on opposing poles for a single phase in a three-phase induction motor are separated.
Japanese Patent Application Publication JP-A-2004-64893 discloses an induction motor wherein two three-phase inverters and two three-phase windings are respectively split and connected. The phases of the two three-phase inverters are 180° out of phase with each other.
Japanese Patent Application Publication JP-A-2006-203957 discloses an induction motor wherein two three-phase inverters and two three-phase windings are each split and connected. A single coil is connected to each phase, where coils that structure single three-phase connections are disposed at 120° angles on the stator, and two three-phase connections are disposed shifted 60° from each other. Two three-phase inverters of an identical phase provide power to the respective three-phase connections.
However, the motors disclosed in the references described above are induction motors, and thus have fundamentally different structures from synchronous motors that use permanent magnets in the rotors.